1. Technical Field
The present invention relates to the field of power electronics. It concerns a method for operating a charger for a voltage-maintaining device, and an apparatus for carrying out the method in accordance with the preamble of the independent claims.
2. Prior Art
Conventional voltage-maintaining devices are presently mainly in use for compensating voltage dips in the system voltage, as a consequence of short circuits or load changes, in an electric AC voltage supply system. Such a voltage-maintaining device is specified, for example, in U.S. Pat. No. 5,329,22. A storage capacitor is connected therein on the DC voltage side to an inverter which is connected on the AC voltage side to a secondary side of a transformer. The primary side of a transformer is inserted into one of the phases of the electric AC voltage supply system. The inverter is driven in such a way that, in the case of a voltage dip in the system voltage, an input terminal voltage is generated which is an AC voltage. This input terminal voltage serves to compensate the dipped system voltage, the generated input terminal voltage being added to the dipped system voltage such that a phase system voltage present at the phase remains uninfluenced and can therefore be maintained without variation. The electric energy required to generate the input terminal voltage is drawn from the storage capacitor, which must be recharged by means of a charger when energy is drawn, the DC voltage across the storage capacitor requiring to be kept as constant as possible.
A suitable charger for a voltage-maintaining device is disclosed, for example, in xe2x80x9cIEEE Transactions on Industrial Electronics, Vol. 46, No. 5, October 1999xe2x80x9d. There, a rectifier of the charger is connected at its phase inputs to the electric AC voltage supply system via a transformer. Furthermore, a control device is provided which serves to generate reference charging voltages, a drive signal for driving power semiconductor switches of the rectifier being formed from the reference charging voltages by means of a driving circuit connected to the control device. The rectifier generates a charging voltage for the corresponding phase at each phase input by switching the power semiconductor switches in accordance with the drive signal. In addition, on the DC voltage side of the rectifier, the rectifier generates a substantially constant DC voltage, and thereby charges the storage capacitor.
A problem with such an operation of a charger is that although the DC voltage can be kept substantially constant on the DC voltage side of the rectifier upon the occurrence of a voltage dip in one of the phases, a charging current which is increased by comparison with the phases not affected by the voltage dip is drawn in the phase with the voltage dip, in order to charge the storage capacitor. However, this increased charging current additionally loads the phase, in particular the phase system voltage already dipped, and so the phase system voltage, which is already reduced in any case, becomes even lower. In the case of a weak electric AC voltage supply system, this can lead, however, to an insufficient supply of electric energy in the case when an electric load is connected to such an AC voltage supply system, as a result of which the electric load can be damaged or destroyed and/or the electric AC voltage supply system must be turned off as a consequence of the excessively low phase voltage in the affected phase. The high degree of availability required of the electric AC voltage supply system is then no longer obtained.
It is therefore the object of the invention to specify a method for operating a charger for a voltage-maintaining device, which method generates a charging current of the charger in a phase, affected by the dip in a phase system voltage, of an electric AC voltage supply system, which charging current does not load the reduced phase system voltage in the affected phase, or produces only an insubstantial extra load. Also specified is an apparatus with the aid of which the method can be carried out in a particularly simple way. This object is achieved by means of the features of claims 1 and 10. Advantageous developments of the invention are specified in the subclaims.
In a method according to the invention for operating a charger for a voltage-maintaining device, a rectifier, connected to phase inputs via a transformer with an electric AC voltage supply system, of the charger is driven by means of a drive signal formed from reference charging voltages, Furthermore, a charging voltage for the corresponding phase is generated at each phase input, a substantially constant DC voltage being generated on the DC voltage side of the rectifier. According to the invention, each charging voltage is generated in such a way that in the corresponding phase a charging current is injected in a fashion substantially proportional to a phase system voltage of the corresponding phase of the electric AC voltage supply system. A particularly advantageous result of this is that in the event of a voltage dip in the phase system voltage, the charging current of the charger no longer loads this phase system voltage, or produces only an insubstantial extra load, such that a further reduction or substantial loading of the phase system voltage already reduced by the dip can be successfully avoided. It is thereby possible to ensure adequate supply of electric energy to an electric load connected to a particularly weak electric AC voltage supply system. In addition, the method according to the invention provides a high degree of availability of electric AC voltage supply system, since there is no generation of an increased charging current as is known from the prior art, which could further reduce the already reduced phase system voltage in such a way that the electric AC voltage supply system must be turned off.
The apparatus according to the invention for carrying out the method for operating the charger for a voltage-maintaining device has a control device which serves to generate the reference charging voltages and is connected via a driving circuit for forming the drive signal from the reference charging voltages to the rectifier. According to the invention, the control device comprises a calculating unit for forming the reference charging voltages, to which phase system voltages of the electric Ac voltage supply system are fed on the input side, a phase system voltage being fed respectively to an impedance-matching device of the corresponding phase. Furthermore, the control device comprises a controller unit for generating a reference phase current amplitude value, to which the DC voltage and a DC voltage desired value are fed, the reference phase current amplitude value being fed to each impedance-matching device on the input side. The apparatus according to the invention for carrying out the method for operating the charger for a voltage-maintaining device can therefore be implemented very easily and cost effectively, since the outlay on circuitry can be kept extremely low and, in addition there is a need for only a low number of components for the design. The method according to the invention can therefore be carried out with particular ease by means of this apparatus.
This and further objects, advantages and features of the present invention become evident from the following detailed description of a preferred exemplary embodiment of the invention, in conjunction with the drawing.